Tomsk State University. Physical Department.
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ACOUSTIC DEVICES

Abstract

The work describes the acoustic lenses and acoustic interference filter used in acoustic antennas. The experimental radiation patterns are presented for an axially symmetric biconvex lens. Also radiation patterns for a lens assembled based on corrugated components are given. A factor of 30 power gain is achieved. A similar gain is obtained by using zone plates with phase inversion. An acoustic Fabry-Perot etalon with resolution up to 2.5% is developed.

Keywords: acoustic lenses,..

In this work we describe lenses and interference filter which, coupled with horns, constitute horn-lens (horn -diffractor) antennas. In these antennas, the horn not only serves to protect the detector from external noise, but also influences significantly the antenna characteristics: the level of side lobes, frequency range, etc. Efficient solution of many atmospheric acoustics problems relies heavily on characteristics of transmitting and receiving antennas: antenna response pattern, gain, and frequency selectivity among others. Until recently, standard acoustic antenna configurations were reflectors (mirrors), multielement (array) systems, and, to a lesser degree, concentrators (horns). Refractors (lenses) and diffractors (zone plates), whose acoustic resistances very differ from that of the air, produce large losses of transmitted power; so they were of only limited utility until recently. Gezekhus of Tomsk State University was first to show (Gezekhus 1890) how this difficulty can be avoided using acoustic devices on the basis of waveguide media.
Acoustic Lens

Of many existing acoustic systems (Kock 1965, Perkalskis et al. 1984), the most efficient devices were found to be lenses and zone plates assembled based on circular waveguides and lenses with corrugated waveguides. The axially symmetric biconvex lens given in Figure 1 is assembled on a cross using coaxial bicones with cone angle ?. The profile of the lens is calculated by following tautochronism principle. The intercone gaps are calculated by condition of most optimal propagation of zero-order wave of frequency f. Experiments show that for short-focus systems with ratio F/D ~ l + 1.5, where F is focal distance, and D the lens diameter, the axially symmetric lenses have minimal spherical aberration, well-defined focal spot, symmetric radiation pattern, and low level of side lobes (Veselovskiy et al. 1988).
Graph

Figure 2 presents experimental radiation patterns at frequency f = 3 kHz for lenses with F = 400 mm and refractive indices n = 1.41 and n = 2; they are obtained for conical horn (curve 1), horn plus lens with n = 1.4 (curve 2), and horn plus lens with n = 2 (curve 3).

By using these lenses together with misaligned horn and a fracture of generatrix, where the phase center location does not depend on the frequency, the level of the side lobes decreased on the average by 3 - 5 dB and at frequency f = 3 kHz composed has reached minus 36 - 40 dB (Root et al. 1988).

cc

The corrugated-waveguide lens configuration is utilized efficiently if it is required to get antenna response pattern with different main lobe widths in azimuth and elevation planes. These antennas are used, for instance, in bistatic acoustic meteorological radars to study near-ground propagation of sound. The plane-spherical lens with F = 400 mm and diameter D = 400 mm was assembled from corrugated plates with cylindrical corrugations radius r = 20 mm (Figure 3). The lens was coupled with a conical horn. The lens gain at the axis is 8 dB at frequency f = 6 kHz.

Later on, to remove the spherical aberration, we fabricated a plane-hyperbolic lens with corrugated waveguides. This lens with the diameter D = 1000 mm, and at frequency f = 6 kHz yields the power gain of received signal up 30 times.

Also, of certain practical interest are cylindrical waveguide lenses. While, on one hand, they provide smaller amplification of received signal than spherical and hyperbolic lenses, on the other, they can be used in combination with multichannel interference detectors, thus permitting fast and quite efficient measurement of distribution of signal phase and amplitude across the antenna opening.

The horn-lens antennas are broadband devices. So, a need arises in certain cases to place a prefilter in lens-detector gap in order to select a narrower frequency band and decrease the negative load on detector. In particular, Fabry-Perot or Kwinke sound interferometers can be used as such filters.

In the modified Kwinke interferometer, one of its U-shaped arms is к wavelengths longer than the other. This modification improves selectivity of the system as its resolution is determined by the product R=?/??=kN, where N is the number of arms in the interferometer. At N=2 and k=20, the resolution will be approximately 2.5%.

Acoustic F-

The acoustic Fabry-Perot interferometer consists of two parallel perforated plates separated by distance l (Figure 5). The incident sound wave strikes the leading interferometer plate, and it is transmitted through the holes between the plates being partially reflected off the back plate. The sound waves flowing out of the device after even number of reflections interfere among themselves. By inter-plate distance l = 250 mm the half-width of spectral line amplitude is 2 - 2.5%. If three plates are used and the distance between them are l1 = 150 mm and l2 = 600 mm, then the half-width of the spectral line amplitude will be on the order of 0.2% (Perkalskis et al. 1990).

References

Gezekhus N.A. 1890, JRPCS, 22, 233

Kock W. 1965, Sound waves and light waves. NY.

Perkalskis B.S., Larin V.L, Sotiriadi G.N. and Mikhailichenko Yu.P. 1984, In: 8th International Symposium on Laser and Acoustic Sensing of the Atmosphere. Part 2, Tomsk, p. 214-216

Veselovskiy et al. Akusticheskii Zhurnal, V. 1988, 34, No. 2, p. 347-348

Root A.G., Perkalskis B.S., Sotiriadi G.N., . Larin V.L, Azbukin A.A. 1988, Atm. Oceanic Opt., V. 1, No. 12, p. 67-68

Perkalskis B.S., Root A.G., Sotiriadi G.N., Larin V.L. 1989, Atm. Opt., V. 2, No. 2, p. 216-217

Larin V.L. 1975, Isv. Vyssh. Uchebn. Zaved. Ser. Fizika., No. 9, p. 149-150

Perkalskis B.S., Ketova T.Y., Larin V.L., Sotiriadi G.N., 1990, Isv. Vyssh. Uchebn. Zaved. Ser. Fizika., No. 1, p. 112-113

 
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